P0172 - Fuel Trim; Bank 1: System Too Rich
P1128 - Fuel Trim; Bank 1 (Mult): System too Lean (Intermittent)

Solution Difficulty:  Moderate

Mass Air Flow or MAF sensors can cause lots of problems on most new cars, but they can usually be fixed by replacing the sensor. This can sometimes be a simple fix, but other times it can require removing parts of the air intake. Since many new cars are rather crowded under the hood, you may want to consult a Haynes or Factory Service Repair Manual before trying this. They can often be full of helpful hints for routine service like this.

We scanned our car, a 1999 VW New Beetle for codes and found that we had two. One was an active fault code P0172 - Fuel Trim; Bank 1: System Too Rich, and another pending code P1128 - Fuel Trim; Bank 1 (Mult): System too Lean (Intermittent). After trying a few of the procedures in our Bentley Service Repair Manual and doing a little research over at NewBeetle.Org we determined that we'd need to replace the MAF, or Mass Air Flow sensor in order to clear the code.

MAF sensors can be an expensive part to replace, luckily our friends over at German Auto Parts had the sensor for a mere $55. (We were quoted as high as $89 w/ $35 core charge at other parts places). The replacement sensor came complete with the plastic housing that holds the sensor in place, and which makes replacement that much easier. For this procedure we only needed a few simple tools including a pair of locking pliers, a 10 mm socket and a screwdriver.

The MAF is located between the air cleaner box and the intake hose. The MAF housing is attached to the air box by two Phillips screws, and attached to the intake hose by a metal clip. As with most cars room under the hood is rare, so it takes a special touch to remove the box. Firs you'll need to remove the two bolts that hold the air box in the engine compartment. You'll also want to remove the rubber grommets as you'll need them later. Then take your pliers and squeeze the metal clip on the intake pipe. This should open up the clip and you can slide it toward the MAF. Pull the intake tube off and release the clip on the MAF. Now rock the air box toward you, and lift up. It should slide out fairly easily.

Once you have the air box out, you can slide the metal clip off the MAF and remove the two screws that hold it in. Now take your new MAF, remove the protective caps and install it on the air box. Make sure not to get any dirt inside the MAF as this could contaminate it and cause it to stop working properly.

Once you have this installed you can slip the clip back on the MAF tube. This will make reinstalling the air box easier. You'll also want to reinstall the rubber grommets if they came loose. It's vastly easier to insert these and then simply bolt them down under the hood. The grommets help absorb vibrations and if you don't hook them up properly you could notice a rattling sound under your hood.

Your now ready to reinstall the air box. The ultimate trick to getting this is to insert the air box at about a 45-degree angle and then rotate it back toward the rear of the car as you lower it. There is a rubber stud on the side of the box that fit into a small metal ring. This helps line up the tube that sends air into the air cleaner. If you get the rubber stud into the ring and slide the box sideways, the tube should line up. You may also need to move the intake tube on the other side of the air box to give you more room. It's flexible, and you can easily bend it to make it fit. Once you've got both sides attached you'll want to slide the metal clip back on to the intake tube and make sure you have a tight seal.

The last bit is to bolt the air box back down. One trick I've found is to attach some tape to the end of your socket. This will help hold the bolt inside if it tries to slip out. This will help avoid dropping the bolt down inside the engine, which can be very frustrating. Once the box is bolted back into the car you can reattach the sensor wire and your ready to go.

At this point you'll need to reset the computer and let the car run a few miles before you'll know if you've fixed the problem. The problem I fixed was due to a long term fuel trim error, so typically it would take a day or two before the CEL (Check Engine Light) would come back on. So far we've driven 200 miles with no problems.

This article contributed by ERSWeb
Read more by this author at http://ElectronicRealitySolutions.com